I've just started a matinee coat using an American pattern. Row 1 of the pattern begins "K12 left front, yo, k1 seam stitch,...". I don't know how to do a seam stitch - I've searched the net and asked some shops and friends but it's not clear. Would anyone be able to help? I'm reasonably ok at understanding knitting stitches.

A seam stitch is usually just an edge stitch that will be used to seam the garment. It usually worked in stockinette. It's sometimes not included in the pattern stitch count, but the pattern usually tells you that. Sometimes it is included in the count and calling it a 'seam stitch' is just defining it as such.